The Red Devils snapped up just one defender over the summer, with Italian full back Matteo Darmian arriving from Torino to aid Van Gaal's defensive options.
Many fans wanted to see a centre back brought in, but United chose not to address that particular area of concern, with Van Gaal sticking with the likes of Chris Smalling, Daley Blind, Phil Jones and Marcos Rojo.
Now though, The Mirror report that Van Gaal is ready to sell Rojo after a bust-up with the Argentina international, and is looking for a replacement ahead of the January transfer window.
It's stated that Netherlands international Martins Indi, who played under Van Gaal at the World Cup last summer, is now a target for the Red Devils, with the United boss keen to engineer an Old Traffor reunion with the centre back.
The 23-year-old, who joined Porto immediately after the World Cup, has featured heavily for the Portuguese outfit since his move from Feyenoord - and could be ideal for Van Gaal given his ability to play at centre back and left back.
A move to the Premier League may be appealing to Martins Indi, whilst a reunion with Van Gaal could also tempt the defender in to a move to Old Trafford - but whether Porto are willing to sell is another matter, and it may take a large bid from United to convince them to sell up in the middle of the season.